1. Field of the Invention
The present invention is directed to the clarification of red mud-containing liquors; particularly the primary settler liquor generated in the Bayer process for the recovery of alumina from bauxite ore.
2. Description of the Prior Art
In the Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore (referred to as "red mud") are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate is precipitated out of the solution and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry then passes through several flash tanks wherein the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 400.degree. F. to about 220.degree. F.
The aluminate liquor leaving the flashing operation contains from about 1 to about 20 weight percent solids, which solids consist of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a "sand trap" cyclone. The finer solid particles are generally separated from the liquor first by settling and then by filtration, if necessary. Any Bayer process slurry taken from the digesters through any subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred hereinafter as the primary settler feed. The slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled wash overflow liquor.
Normally, the primary settler feed is thereafter fed to the center well of the primary settler, where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution, referred to as "green" or "pregnant" liquor, overflows a well at the top of the primary settler and is collected. This overflow from the primary settling tank is passed to the subsequent process steps.
The clarity of the primary settler overflow is crucial to the efficient processing of alumina trihydrate. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 10 to about 5,000 mg of suspended solids per liter), it must be further clarified by filtration to give a filtrate with no more than about 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before alumina trihydrate is recovered is referred to as a secondary clarification stage.
The clarified sodium aluminate liquor is seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH).sub.3. The alumina trihydrate particles or crystals are then separated from the concentrated caustic liquor, and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
In another section of the Bayer circuit, the settled solids of the primary settler ("red mud") are withdrawn from the bottom of the settler ("underflow") and passed through a countercurrent washing circuit for recovery of sodium aluminate and soda. Overflow water from this washing circuit is recycled as primary settler feed and dilutes the slurry as it leaves the flash tanks. The efficient recovery of soda and sodium aluminate is one important way operators reduce costs and increase production.
The partial separation of the red mud solids from the pregnant liquor in the primary settler is expedited by the use of a flocculant. This initial clarification of the pregnant liquor is referred to as the primary settler stage. Flocculating agents, such as inverted latex polymers and starch, improve the separation of insolubles by increasing the rate at which the solids settle, by reducing the amount of residual solids suspended in the liquor, and by decreasing the amount of liquor in the settled solids phase. Flocculation performance is highly important in the primary settlement stages. Red muds are comprised chiefly of iron oxides (at least about 50 weight percent of the red mud solids), together with silicon oxides, calcium oxides, sodium alumino-silicates, titanium oxides and other material, and commonly represent from about 5 to about 50 weight percent of the materials of the bauxite ore. Generally these muds are comprised of very fine particles, which hinder the desired rapid and clean separation of red mud particles from the solubilized alumina liquor. If the rate of separation is slow, output is materially diminished and overall process efficiency is impaired. If the separation is not clean, the resultant solubilized aluminate liquor will require a more extensive treatment to remove residual solids, and/or the alumina trihydrate recovered will contain levels of impurities that are undesirably high for many end-uses.
Starch and inverted latex polymers are used to flocculate the suspended red mud. These flocculating agents are added in the primary settler. Starch is added to improve the clarity of the liquor overflow. However, starch disadvantageously produces a red mud which has poor rheology, and, hence, has undesirable underflow characteristics. In more detail, the red mud is removed to a wash stage via an underflow mechanism. In the wash stage, sodium aluminate and soda are recovered from this red mud. When starch is used, a mud is produced which has poor rheology and poor underflow characteristics. If not diluted, the underflow will damage plant equipment. To overcome this problem, operators have decreased the percent solids in the underflow to make it more manageable. This, however, deceases the efficiency of the soda and sodium aluminate recovery in the wash stage.
When latex polymers are used, the latex polymers must be diluted either at the process facility or prior to shipping. If the latex polymers are diluted prior to shipping, shipping costs are dramatically increased. If the latex polymers are diluted at the processing facility, equipment must be available at the facility to perform the dilution. Furthermore, other than the dilution step, the latex polymers must be inverted prior to application so that the latex polymers are contained in the water phase of the emulsion. If latex polymers are added neat (non-inverted), the non-inverted, oil phase latex polymers will not disperse in the Bayer liquor, and the oil phase will plug injection equipment. Accordingly, in light of the above problems with latex polymers, it would be advantageous to either eliminate the need for latex polymers or decrease the amount of latex polymers needed to adequately flocculate red mud solids in a Bayer process liquor.
Accordingly, it would be advantageous to provide a new flocculating agent which would expedite the separation of the red mud solids from the pregnant liquor, increase the clarity of liquor overflow, decrease latex flocculant usage, and eliminate starch.